1. Field of the Invention
The present invention relates to an electromagnetic clutch for transmitting a motive power of an engine or the like of a vehicle to a drive shaft of a compressor used in an air conditioner for the vehicle.
2. Description of the Related Art
A compressor of this sort is driven, for example, by receiving a motive power of an engine. For this reason, the engine and a drive shaft of the compressor are connected to each other through the motive power transmitting path. The driving force of the engine is transmitted from a driving belt to a pulley and then transmitted from the pulley to the drive shaft of the compressor via an electromagnetic clutch through this motive power transmitting path. More specifically, the electromagnetic clutch includes a rotor which is rotated about the drive shaft. An armature, i.e., a clutch plate, is disposed so as to face the rotor. The rotor attracts the clutch plate by causing a current to flow through an electromagnetic solenoid, thereby transmitting the driving force to the drive shaft.
Here, when galling or the state of a lock is caused in the drive shaft, the driving belt or the like is damaged, which leads to a stop of the engine. Thus, an electromagnetic clutch is disclosed in which the heat generation due to skid of the clutch plate is detected with a thermal fuse or the like and the connection between the clutch plate side and the drive shaft side is cut off. Also, an electromagnetic clutch provided with a torque limiter is disclosed for cutting off connection between the clutch plate side and the drive shaft side when the state of a lock is caused (refer to Japanese Unexamined Patent Publication No. Hei 8-135686 and Japanese Patent Publication No. Sho 58-1294 for example).
Now, in the case of the above-mentioned electromagnetic clutch provided with the fuse or the like, since it takes a certain period of time to detect the skid of the clutch plate, problems such as damage to a housing of the compressor, or transmission of the frictional heat to the housing occur during that period of time, leading to the lowering the sealing property.
In addition, when an elastic member made of rubber or the like is used to perform the function of the above-mentioned limiter in the above-mentioned electromagnetic clutch, changes over time such as reduction in the tension or elastic force of the rubber are feared, and there is a problem that when the drive shaft is in a heavy lock state, it may be difficult to cut off the connection between the clutch plate side and the drive shaft side.
Moreover, in the case of the above-mentioned electromagnetic clutch provided with the torque limiter, the recovery of the connection between the clutch plate side and the drive shaft side is usually tried. That is to say, in addition to the case where the drive shaft is in a light lock state, even in the case where the drive shaft is in a heavy lock state and the heavy lock state is not released, the recovery of the connection between the clutch plate side and the drive shaft side is made. In this case, there is a problem that when an internal component or part of the compressor is damaged along with the recovery of the connection between the clutch plate side and the drive shaft side, the internal component or part is dragged to cause a crack in the housing, and the housing is also damaged.
Here, in recent years, as a refrigerant of the air conditioner for a vehicle, natural refrigerant have been developed and studied from a viewpoint of protection of the ozone layer and prevention of the global warming. A combustible refrigerant such as a CO2 or alternate from HFC-152a can be mentioned as the refrigerant. In particular, in the case of the CO2, the Global Warming Potential (GWP) of the refrigerant is 1. In addition, in the case of the HFC-152a, the GWP of the refrigerant is about 1/10 times as small as that of HFC-134a, which greatly contributes to the reduction of the environment load.
However, when the refrigerant concerned is discharged from the compressor into the atmosphere due to the damage of the housing or the reduction of the sealing property, in the case of the CO2 refrigerant, if a predetermined concentration is exceeded, this may exert a bad influence on the human body. In addition, the combustible refrigerant may catch fire. For this reason, it is necessary to take measures for avoiding the discharge of the refrigerant from the compressor into the atmosphere. However, the prior art does not specially take this point into consideration.